This invention relates to an anti-skid control device and, more particularly, to an anti-skid control device incorporating a microcomputer for controlling a skid. Such anti-skid control device is for the purpose of automatically releasing a braking force applied to wheels of a vehicle during braking operations to prevent wheel locking and thus to insure a safe braking operation.
Generally, an automobile is steerable due to friction between road surfaces and wheels, and can run safely as long as a frictional force, or a clinging force, of the wheels is maintained at a suitable value. However, if a force exceeding the clinging force of the wheels acts on the automobile, the wheel or wheels slip on the road surface preventing a normal running of the automobile and causes abnormal running conditions. This condition is generally referred to as "skid".
The skid is affected by various factors. One of these factors is a speed of the automobile. A high automobile speed tends to cause a spin of the automobile even at a small steering angle. Abrupt increases in a yawing angle velocity and in a slip angle of rear wheels are observed in a state of spinning. The skid condition is also affected by a frictional coefficient of the road surface. For instance, comparing a concrete road surface (frictional coefficient 0.8) with a wet road surface (frictional coefficient 0.4), the spin takes place at a smaller steering angle in the case of the road surface with a small frictional coefficient, i.e. the wet road surface, than in the case of the road surface with a higher frictional coefficient.
A typical and conventional anti-skid controlling method is to compare a wheel speed with a estimated vehicle velocity by using a comparator and to control the brake release valve successively by the output of the comparator.
A device of the type described above is shown in U.S. Pat. No. 4,094,556, for example.
However, the device of the prior art for an anti-skid control device has had the disadvantage that a considerably long time lag is observed for the anti-skid control because it is time consuming to perform a continuous calculation of slip ratio from wheel velocity and estimated vehicle velocity to determine the timing of the brake releasing operation.